Such methods and devices are known from the prior art and can be used in particular in the context of investigating material properties under predetermined and, as the case may be, variably predetermined ambient conditions. Examples of this are the investigation of the sorption capacity, the (e.g. thermal) length expansion, the rigidity, the crystallographic structure etc. of a material sample, which is arranged in a sample chamber through which the carrier gas/vapour mixture stream flows.
The carrier gas can in particular be an inert gas (e.g. nitrogen, helium, argon etc., or mixtures of noble gases). The vaporous component or the liquid whose vapour is contained in the carrier gas/vapour mixture stream is usually water, but mixtures with vapours of organic components such as for example alcohols are known.
The following methods for the generation of carrier gas/vapour mixtures with the aim of generating a predetermined vapour concentration (equivalent to predetermined proportions of the carrier gas and the vapour) are established in the prior art:
evaporation of the liquid and making the carrier gas available, and subsequent metering/mixing of the two components in the gaseous state.
Evaporation of a previously metered liquid stream and addition of a metered carrier gas stream (so-called “direct evaporation method”).
Mixing of a carrier gas stream saturated with the vaporous component and a pure carrier gas stream.
Generation of a carrier gas stream saturated with the vaporous component and targeted condensing-out of excess vapour at a heat exchanger surface.
The metering of all the components in the gaseous state is bound up with considerable technical difficulties, since the mass flow measurement has to be carried out at raised temperature to avoid re-condensation. In the case of water as a vapour component, the temperature must for example be greater than 100° C. Many commercially available mass flow meters are thus ruled out, i.e. expensive special solutions for high media temperatures have to be selected.
The precision in the achievement of a target concentration of the vapour is limited in the method of direct evaporation by the metering accuracy of the liquid component. For example, the liquid mass flow is often so small that it cannot be metered with sufficient accuracy (“slow-flowing”) and/or cannot be evaporated uniformly. A further drawback of direct evaporation is the relatively long rise and, in particular, fall times of the vapour concentration with a change in the metered liquid quantity between zero and a target value different from zero (typically in the region of several minutes).
The methods in which a carrier gas stream saturated with the vaporous component is first produced are disadvantageous in that a pure vapour atmosphere (vapour concentration 100%) and also a concentration close to 100% cannot be achieved on account of the required participation of carrier gas.
The methods of mixing a saturated and a vapour-free carrier gas stream are essentially limited to the use of water as a vaporous component, since it is only in this case that the measurement of the vapour concentration can be carried out in a particularly straightforward manner (e.g. by means of commercially available sensors for air humidity).
It is the problem of the present invention to overcome the aforementioned drawbacks of the prior art and to provide a method and a device for the generation of a continuous carrier gas/vapour mixture stream, by means of which the concentration of the vapour component can be adjusted in a wide range with little delay and high precision.